1. Field of the Invention
The present invention is related to the electrophotographic system for forming an image corresponding to the image of an original or the image of informations entered on a recording medium.
2. Description of the Prior Art
According to an electrostatic photo-copying machine, a laser printer or the like, optical information such as an image of an original or character data to be printed is projected onto a recording medium having a photoconductive layer, having been electrified or sensitized uniformly, to form an electrostatic latent image thereon. The latent image is developed with toner as a coloring agent and the image developed is transferred onto a copy paper. After the transfer of the image, toner still remaining on the recording medium is removed in order to use the recording medium repeatedly.
As a photo-conductive material for forming an electrostatic latent image on the photo-conductive layer of the recording medium, there is used an inorganic photoconductive material such as Se, CdS or ZnO, or an organic photo-conductive material such as PVK-TNF (polyvinyl carbozole-2,4,7-trinitro-9fluorenone).
None of these materials satisfy all of the conditions being required as a photoconductive material such as a high photo-sensitivity, a high spectroscopic photo-sensitivity, a high S/N ratio, a good durabilty and/or a pollution freedom for the human body. Under such a situation, one of them is relatively chosen according to the object of the application.
On the contrary to the above, amorphous silicon (hereinafter referred to a-Si) has been developed as a photo-conductive material since it is expected to have advantages such as a high photo-sensitivity, a high durability, a pollution freedom, etc.
However, it has disavantages as follows. One of them is that the band through which photo-carriers are moved has a skirt portion having a distribution of an exponential function at its end which gives undesirable effects upon forming the image.
FIG. 5 shows a conventional photo-copying system schematically.
Around a photo-conductive drum 1 having a photoconductive layer of a-Si, there are arranged a main charger 2 for electrifying the surface of the drum 1 into a predetermined polarity uniformly, an optical means 3 for focusing the reflected image of the original on the photoconductive layer to make an electrostatic latent image, a developing means 4 for developing the electrostatic latent image formed on the drum 1 with toner, a transfer means 5 for transferring the developed toner image onto a transfer sheet 6, a cleaning device 7 for removing residual toner on the drum and a photo-potential eraser (discharger) 8 for erasing the surface potential remaining on the surface of the drum by applying light thereonto.
In the copying system mentioned above, photocarriers are captured in the skirt portion of the band mentioned above which have been generated excessively upon charging and exposuring by the main charger 2 and the optical means 3, respectively. These carriers will disappear by recombination therein directly or after being excited up to the band when they are left alone for a relatively long time. But, when the charging and exposure are repeated in a short time interval, carriers having been captured are urged to pass through the skirt portion of the band or the excited band by a strong potential generated upon charging and, then, cancel charges on the surface of the drum since each of them has a polarity opposite to that of the surface charge. Due to this, the surface potential of the drum is lowered to cause undesirable effects upon forming the image. This effect is enhanced with respect to a-Si, since both an electron and positive hole are movable therein.
Further, if the spectrum of the light used for exposure is different from that of the light used for erasing the surface potential, the distribution of carriers in the skirt portion will become different at the exposure and at the erasing of the surface potential. This causes memory effects since a level difference in the photo-fatigue is caused at the next main charging process. Therefore, the image is greatly affected due to memory affects to cause an irregular copy.